As far as trolling motor power get the biggest motor you can find within your budget....
i love my 74lb thrust on my 18 ft rig and it hauls!I have 80# on my 17.5" boat and it hauls with TM. I wouldn't go lower than 70# though. You want to be able to chase down those boils.
Your '89 TM aint worth didley. Just give it to someone or throw it in the garbage.
i have my 74 on 40 all day unless im going in to get my sis line un snagged or chaseing the sag boiles!I have a 55lb on a 16 ft skeeter, and will be going with at least a 70lb next time. Put it this way, I never have the 55lb on anything other than max speed, and there are many times I wish I had more go!! As fast as you can afford!